The present invention relates to a method and an apparatus for measuring a displacement of a sample by use of light interference, more particularly to a method and an apparatus for measuring a displacement in which a sample is irradiated with laser light, reflected light is allowed to interfere with reference light, and a displacement of the sample is measured from the resultant interference signal.
As a method for measuring a displacement or a movement of a sample, a method using light interference has been broadly known (Meas. Sci. Technol., 9 (1998), 1024 to 1030). One example is shown in FIG. 10.
In an interferometer shown in FIG. 10, a laser head 301 emits double-frequency orthogonally polarized light beams 302 whose polarization directions cross each other at right angles and whose optical frequencies are different from each other by 20 MHz. The light beams are split into two polarized components by a polarization beam splitter 303. After an S-polarized light beam 303′ is reflected by the polarization beam splitter 303, the light beam is reflected by a right angle prism 304, and enters the polarization beam splitter 303 as reference light. A P-polarized light beam 305 passes through the polarized light beam splitter 303. The light beam is reflected by a right angle prism 306 placed on a sample to be measured 400, and enters the polarization beam splitter 303. Both of the reflected light beams are combined in the polarization beam splitter 303, and pass through a polarizing plate 307 having a polarizing angle of 45° with respect to polarization directions of both of the reflected light beams to cause heterodyne interference.
This heterodyne interference light is received by a photoelectric conversion element 308, and is converted into an electric signal 309. Doppler shift frequency is added to a frequency fM of the heterodyne interference signal 309 in accordance with a moving velocity V of the measurement sample 400, and the frequency fM is given by Equation 1:fM=fB±NV/λ  (1),where fB=20 MHz, λ denotes a wavelength of laser light, and N=2, 4, . . . which denotes a constant determined by the number of times the light travels both ways in an optical path. In FIG. 10, N=2. On the other hand, a beat signal 310 indicating fB=20 MHz is output as a reference signal from the laser head 301.
The measured heterodyne interference signal 309 and reference signal 310 are input into a phase detection circuit 311, the moving velocity V and a movement 400d of the measurement sample 400 are obtained from a phase difference between both of the signals, and a movement output signal 312 is output.
In the interferometer shown in FIG. 10, a probe optical path, that is, an optical path through which the P-polarized light beam 305 as probe light passes is spatially separated from a reference optical path through which the S-polarized light beam 303 as the reference light passes. Therefore, when a temperature or refractive index distribution is made by a fluctuation of air or the like, or a mechanical vibration is generated, an optical path difference varies between both of the optical paths, and this generates a measurement error of a nanometer order. A positioning precision of the order of a sub-nanometer or less is required in an exposure device for manufacturing a semiconductor fine pattern for a 45 nm or 32 nm node in future, a stage of a pattern dimension measurement apparatus, or a probe microscope for use in local characterization. The conventional technique shown in FIG. 10 cannot meet the requirement. There is supposed a method of controlling environment factors such as temperature, humidity, and mechanical vibration with a high precision, but economical effects remarkably drop with regard to apparatus costs and sizes, and conveniences.